A traditional solid-state detector type to be used for X-ray detection in e.g. imaging and spectroscopy applications is the PIN detector. There the detecting element is a reverse biased PIN diode, one electrode of which is coupled to the gate of a FET (field-effect transistor). X-ray photons that hit the PIN diode cause a photoelectric effect, creating a number of free electrons and holes in a depleted region formed in the semiconductor material. The bias voltage across the PIN diode causes the mobile charge carriers to be drawn to the electrodes, which changes electrode potential. An integrator coupled to the FET transforms the change of the PIN diode's electrode potential into a corresponding change in a voltage across a feedback capacitor.
A drift detector is a more advanced detector type, which has been described in detail for example in the publication C. Fiorini, P. Lechner: “Continuous Charge Restoration in Semiconductor Detectors by Means of the Gate-to-Drain Current of the Integrated Front-End JFET”, IEEE Trans. on Nucl. Sci., vol. 46, No. 3, June 1999, pp. 761–764. The solid-state semiconductor detector element of drift type detectors is most commonly made of silicon, for which reason these detectors are very commonly referred to as SDDs (Silicon Drift Detectors). An SDD, a partially cut-out example of which is illustrated in FIG. 1, differs from a conventional silicon-based PIN diode detector in that it has a field electrode arrangement comprising the so-called drift rings 101 and an amplifier integrated with the diode component. The amplifier is most typically a FET (Field-Effect Transistor), the source, gate and drain electrodes appear in FIG. 1 as 102, 103 and 104 respectively. The anode and cathode of the detector diode are illustrated as 105 and 106 respectively in FIG. 1.
FIG. 2 illustrates schematically the elecrical operating principle of an SDD according to FIG. 1. The dashed elliptical line illustrates, which part of the circuit diagram are located directly within the detector chip. An X-ray photon that hits the detector generates a cloud of free charge carriers, the size of the cloud—and correspondingly the total electric charge it contains—being dependent on the incident energy of the photon. The internal electric field of the detector causes the free charge carriers to drift towards the anode and cathode electrodes. The electric charge represented by the electrons arriving at the anode A can be thought of as a current pulse iq. The detector capacitance acts as an integrator that collects the current pulses and integrates them into an accumulating voltage. Said detector capacitance consists mainly of the anode capacitance Ca, but necessitates also considering the stray capacitances of the FET. The last-mentioned are illustrated as Cdg and Cgs in FIG. 2. Since the circuit arrangement makes the FET operate as a source follower (meaning that the gate-to-source voltage Vgs remains constant), only the drain-to-gate capacitance Cdg makes a true contribution to the detector capacitance Cdet:Cdet=Ca+Cdg  (1)Assuming that the charge collected at the anode as a consequence of the hit of a single X-ray photon was q0, we may write the expression for a change ΔVs in the potential Vs:ΔVs=q0/Cdet=q0/(Ca+Cdg)  (2)which may be expressed in other words so that the charge-to-voltage conversion factor of the detector is (Ca+Cdg)−1.
In the absence of any charge neutralization mechanisms the detector would quickly saturate as collected charge builds up on the anode. Continuous operation is made possible by a leakage current IL, which occurs in the channel of the FET as a consequence of so-called impact ionization and acts to neutralize the accumulating charge. A description of various mechanisms related to the generation of the leakage current IL is found in the publication E. Elad: “Drain Feedback—a Novel Feedback Technique for Low-Noise Cryogenic Preamplifiers”, IEEE Trans. on Nucl. Sci., NS-19, No. 1, 1972, pp. 403–411. The ionization rate, which is a major factor controlling the flow of the leakage current IL, depends strongly on the voltage Vdg across the drain-gate junction of the FET. We may deduce that the value of the leakage current IL depends at least partly on the drain-source voltage Vds of the FET in the SDD.
Under constant illumination by X-rays the repeatedly occurring radiation-induced current pulses to the anode draw the anode potential into the negative direction as long as steady state conditions are reached, under which the leakage current IL becomes equal to the radiation-induced current. As we noted above, Vgs stays constant due to the source follower property of the circuit, so the change in the anode potential can be directly observed as a change in Vds. A detector appliance utilizing the detector described above with reference to FIGS. 1 and 2 comprises a voltage-sensitive measurement arrangement (not shown in FIG. 2) adapted to measure changes in the potential designated as Vs. Since the relative magnitude of the leakage current IL compared to the total current through the FET is negligible, it has become customary to designate the constant current drawn from the source of the FET to a fixed negative potential as Id even if literally taken a more appropriate designation could be Is.
A problem of conventional SDD-based detector appliances is the rate dependent shift in peak positions. When the rate at which X-ray photons hit the detector increases, also the voltage Vds increases. This would not be a problem as such, but becomes one because the detector capacitance is not constant but depends on voltages within the detector chip. A change in the detector capacitance Cdet means a change in the charge-to-voltage conversion factor of the detector. In a measured X-ray spectrum a peak representing radiation at a certain constant energy will shift to different locations depending on whether X-ray photons of that energy arrived at a slower or a faster rate. Whether the shift is upwards or downwards depends on the mutual order of relevance of the capacitances Ca and Cdg, because these have oppositely directed dependencies on the photon hit rate.
Since the change in the charge-to-voltage conversion factor is essentially a consequence of a change in the voltage Vdg, a person skilled in the art might consider compensating for it by changing the drain potential Vd as a function of momentary photon hit rate. FIG. 3 illustrates a possible solution following this approach. A drift-type detector chip 301 comprises a FET 302, the source of which is coupled to a signal output through a preamplifier coupling 303. There is also a current generator 304 adapted to draw a constant current through the FET; as noted above, since this current is essentially the same as the drain current, it is conventional to designate the current drawn by the current generator 304 as the drain current Id, although the coupling is to the source electrode of the FET.
A differential amplifier 305 is coupled to measure the drain-to-source voltage Vds across the FET 302 and to give an output proportional thereto. A controlling amplifier 306 is adapted to compare the output of the differential amplifier 305 to a fixed reference voltage Vref and to use the comparison result to change the voltage produced by a controllable voltage source 308. The amplification and polarity of the controlling amplifier 306 have been carefully tuned so that the resulting change in Vd compensates for the rate-induced change in the charge-to-voltage conversion factor. Since the change of the charge-to-voltage conversion factor is nonlinear, a linearization circuit 307 is needed between the controlling amplifier 306 and the controllable voltage source 308.
The circuit of FIG. 3 involves problems that mainly relate to its susceptibility to oscillations. The most critical part is the differential amplifier 305, the input of which must be very well balanced on the operational frequencies of the control loop. It may also prove to be difficult to realize the linearization circuit 307, because of the complexity of the nonlinear behaviour of the charge-to-voltage conversion factor. The circuit may also be somewhat slow to react to changes in the photon hit rate.